If using the LCD screen, I would put all 4 on the scale, and remove one until one took off more weight than the others. only using the scale once. Or you could go the opposite route, and you would have to put them all on the scale one at a time as to not spill. So just watch the weights as you put a new cup on and you would know which one had the poison.
If you are saying actual months like you obviously did with seasons, then most months do not have an "n". If you are going off that sentence, then you could also say "o" because summer does not have an "o"